I am proposing to examine the nature and distribution of a morphogenetic factor that is suspected of being important in the outgrowth and anteroposterior polarization of the developing limb. The distribution of this activity will be examined in mutant mouse and chick embryos and in the normal mouse. An attempt will be made to characterize the morphogenetic factor with the use of an in vitro bioassay devised that measures its ability to maintain thickened limb bud apical ectodermal ridge. The factor will be examined both in homogenates of limb tissue possessing activity and in tissue culture medium in which the tissue has been cultured. BIBLIOGRAPHIC REFERENCES: J.A. MacCabe and B.W. Parker (1976). Polarizing activity in the developing limb of the Syrian hamster. J. Exptl. Zool. 195:311-317. J.A. MacCabe and B.W. Parker (1976). Evidence for a gradient of a morphogenetic substance in the developing limb. Develop. Biol. 54:297-303.